1. Technical Field
The embodiments described herein relate to a receiver in a communication system and more particularly to a receiver in a communication system for reducing program clock reference (PCR) jitter.
2. Description of the Related Art
In the field of data communications, one highly demanding application is transmission of full motion video in video-on-demand applications. Digital Video Broadcasting-Terrestrial (DVB-T) standard is a DVB European consortium standard for broadcast transmission of digital terrestrial television. This system transmits a compressed digital audio/video stream using OFDM modulation with concatenated channel coding (i.e. COFDM). The adopted source coding method, for example, is Moving Picture Experts Group (MPEG) standard, number 2 (commonly called “MPEG-2”), as defined in standard ISO/IEC 13818-1: 1996 promulgated by the ISO/IEC.
A MPEG-2 transport stream involves transmission of video and audio information in transport stream packets of 188 bytes in length. Each packet includes a header, which contains control information, and a payload, which contains video or audio information. The MPEG-2 transport stream may carry multiple different programs simultaneously. Each packet in the transport stream is associated with a program by a packet identifier (PID) contained within the header. The header is of variable length, depending on whether it contains an adaptation field. The adaptation field contains control information that is not necessarily present in every transport stream packet.
Periodically, the transport packet for each program includes a program clock reference (PCR) value within the optional adaptation field. For example, the PCR may be present in only 10 out of every 4000 video transport packets. The PCR is a representation of a system time clock that was used at the source to encode the data in the transport stream packet. PCR values for a particular program are received with different transport stream packets having the same PID. A series of PCR values can be used to reconstruct the system time clock at a receiver in a communication channel.
The MPEG-2 transport stream may, for example, be transmitted through asynchronous transfer mode (ATM) networks developed for high speed, packetized digital transmission of data, audio and video. One of the parameters used in characterizing an MPEG-2 transport stream is PCR jitter. In an ideal communication channel, all transport stream packets, and therefore all PCR samples, are received at a fixed time after transmission. However, in a real communication channel, variable delays may be introduced by different channel elements. For example, different transport stream packets may follow different network paths in reaching the final destination. Variations in arrival times result in PCR jitter. PCR jitter is described in standard ISO/IEC 13818-9: 1996.
MPEG receivers synchronize internal clocks to received PCR values by sampling the PCR values. If the PCRs do not arrive with sufficient regularity, then the regenerated STC may jitter or drift. The greater the PCR jitter, the harder it is to recover an accurate clock in a MPEG receiver. The MPEG receiver may even go out of lock.
Conventional technology for reducing PCR jitter is using an extra PCR buffer for storing demodulated symbols and outputting the same in a regular order. However, the PCR buffer size requirements can be large, for example, 20*199*8*2=60160 bits in worst case of 64 QAM and code rate equal to ⅞, resulting in high cost in VLSI implementation. Therefore, a low-cost solution for PCR jitter reduction in a receiver is needed.